This application is based on application No. 11-216178 filed in Japan on Jul. 30,1999, the content of which incorporated hereinto by reference.
The present invention relates to a discharge method which can efficiently discharge a plurality of rechargeable batteries, and to a battery assembly containing a discharge control circuit which discharges batteries according to this discharge method.
A battery assembly can output high voltages by connecting a plurality of rechargeable batteries in series. When this battery assembly supplies power to a load, the same current flows through all the rechargeable batteries. For this reason, if the capacities of all the batteries are not equal, i.e. unbalanced, the remaining battery capacity of a low capacity battery will become smaller than that of a high capacity battery. As a result, when the high capacity battery becomes completely discharged, the low capacity battery has already over-discharged. This problem can be overcome by stopping battery assembly discharge when the low capacity battery becomes completely discharged. However, when battery assembly discharge is stopped in this fashion, the total power output from the entire battery assembly is limited by the battery with the lowest battery capacity.
This problem would not occur if the battery assembly could be charged and discharged in a manner that created no battery capacity imbalance among any of the rechargeable batteries. However, connecting a plurality of rechargeable batteries in series and keeping all battery capacities equal at all times is, for practical purposes, utterly impossible. As charging and discharging is repeated, the battery capacities of a plurality of series connected rechargeable batteries have the tendency to become more unbalanced.
The imbalance between individual batteries of a battery assembly becomes more of a problem as the number of series connected batteries increases. For example, in a battery assembly with 100 rechargeable batteries connected in a series, keeping all rechargeable batteries in the same charge capacity state while charging and discharging repeatedly, is extremely difficult. For such a battery assembly, electrical performance of the system is significantly degraded if the capacity of even one battery is low. This is because the power which can be output by the battery assembly is relatively low when discharging is stopped to prevent over-discharge of the low capacity battery as described above. On the other hand, if discharging is continued to increase battery assembly output until the low capacity battery is over-charged, the efficiency of that battery rapidly deteriorates and the performance of the battery assembly as a system is degraded.
As a solution to this problem, a battery assembly provided with an auxiliary battery, separate from the series connected batteries, was developed. In this battery assembly, the remaining battery capacity of each battery is measured during discharge, and power is supplied from the auxiliary battery to the battery with the lowest remaining battery capacity. Since the battery with the lowest remaining battery capacity is supplied with power from the auxiliary battery, over-discharge is prevented. Therefore, total battery assembly output is increased while preventing over-discharge of the low capacity battery.
Although a battery assembly provided with an auxiliary battery can prevent over-discharge of the low capacity battery, it is necessary to include an additional auxiliary battery which is not used at all times. Consequently, as the size of the battery assembly increases, manufacturing cost increases, and since the auxiliary battery is not used at all times, the battery assembly has the drawback that the auxiliary battery cannot be used sufficiently. This battery assembly has the further drawback that over-discharge of the low capacity battery cannot be prevented if auxiliary battery capacity becomes low. The lifetime of the auxiliary battery may not always be greater than the lifetimes of all the batteries comprising the battery assembly.
Further, selection of the battery capacity for the auxiliary battery is extremely difficult. The purpose of the auxiliary battery is to compensate for battery capacity imbalance between a plurality of rechargeable batteries and prevent over-discharge of the low capacity battery. As battery capacity imbalance increases, a higher capacity auxiliary battery is required. However, the imbalance between individual batteries is definitely not fixed and varies widely depending upon the battery assembly. If auxiliary capacity is made large to reliably compensate for battery capacity imbalance, over-discharge of the low capacity battery can effectively be prevented, but auxiliary battery utilization efficiency decreases. In contrast, if auxiliary battery capacity is made small, the auxiliary battery cannot reliably prevent over-discharge of the low capacity battery.
The present invention was developed to eliminate these types of drawbacks associated with prior at battery assemblies. Thus, a primary object of the present invention is to provide a method of discharging a plurality of rechargeable batteries and a battery assembly which discharges according to that method wherein over-discharge of the low capacity battery can be prevented and the total output of the battery assembly can be increased without using an auxiliary battery.
The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.
The discharge method of the present invention is a method of discharging a plurality of rechargeable batteries connected in a series, wherein the remaining battery capacity of each discharging rechargeable battery is measured, and discharge proceeds while battery voltage stepped down from the output voltage of a plurality of series connected rechargeable batteries is supplied to a rechargeable battery determined to have low remaining charge.
In this discharge method, remaining battery charge can be determined by measuring the voltage of each rechargeable battery. The remaining battery charge of a rechargeable battery with reduced battery voltage is judged as low, and thus the low charge rechargeable battery can be determined.
In the discharge method of the present invention, output voltage from the series connected rechargeable batteries is converted to a battery level voltage by a DC/DC converter, and this voltage can be supplied to the low capacity rechargeable battery. The DC/DC converter reduces voltage from a level capable of charging the low capacity rechargeable battery to a level which reduces low capacity rechargeable battery discharge current. DC/DC converter output voltage is supplied to the low capacity rechargeable battery.
DC/DC converter output voltage can be changed depending on the remaining battery capacity of the low capacity rechargeable battery.
Further, the battery assembly of the present invention contains a plurality of series connected rechargeable batteries and a discharge control circuit to control the discharge state of the rechargeable batteries. This battery assembly discharges a plurality of series connected rechargeable batteries to supply power to a load. In addition, this battery assembly has a discharge control circuit provided with a remaining battery capacity detection circuit to measure the remaining battery capacity of each rechargeable battery and determine the low capacity rechargeable battery, a DC/DC converter to step down the output voltage of a plurality of series connected rechargeable batteries to a battery level voltage, and switching circuitry to supply DC/DC converter output to the low capacity rechargeable battery. This battery assembly discharges while supplying output voltage, from a plurality of rechargeable batteries converted to battery level voltage by the DC/DC converter, to a rechargeable battery determined to have low remaining battery capacity by the remaining battery capacity detection circuit.
In this battery assembly, DC/DC converter output voltage can be changed depending on the remaining battery capacity of the low capacity rechargeable battery.
The above discharge method, and battery assembly utilizing that method for discharging rechargeable batteries have the outstanding feature that over-discharge of each rechargeable battery can be prevented while total output can be increased, even though an auxiliary battery is not used. Thus is because a rechargeable battery with low battery charge, which has the possibility of being over-discharged, is determined to be the low charge rechargeable battery, and power is supplied to this low charge rechargeable battery from the other batteries.
A discharging method and battery assembly with many rechargeable batteries connected in series, which can discharge without over-discharging any individual battery, can not only increase battery assembly total power output, but also can extend the life of the rechargeable batteries. This discharge method and battery assembly have the exceptional characteristic that each rechargeable battery can be discharged under ideal, balanced conditions. This characteristic is extremely important for a battery assembly, such as an automobile battery assembly, which has a large number of serially connected batteries. This is because the probability that one rechargeable battery will degrade increases in proportion to the number of batteries in a battery assembly with many serially connected rechargeable batteries, and if a battery degrades, the entire battery assembly becomes unuseable. Further, since the overall cost of a battery assembly with many serially connected rechargeable batteries is extremely high, long usable lifetime is important. Nevertheless, if any rechargeable battery in this type of battery assembly degrades and become unusable, the many other rechargeable batteries, which are in usable condition, become unusable. Therefore, in a battery assembly with many serially connected rechargeable batteries, having ideal discharge conditions for each rechargeable battery is of utmost importance.
Moreover, the discharge method and battery assembly described above do not prevent over-discharge of a degraded rechargeable battery via an auxiliary battery. Over-discharge of a degraded rechargeable battery is prevented via the other non-degraded rechargeable batteries. For this reason, there is no need to provide an auxiliary battery. Consequently, there is no need to control auxiliary battery charge and discharge, and the feature that all rechargeable batteries can be discharged under ideal, balanced conditions is realized.